There are some tests that have to use real clusters but most scenarios can be simulated with simulacron.
Example tests that can be moved:
MetricsTests except for Should_RemoveNodeMetricsAndDisposeMetricsContext_When_HostIsRemoved (maybe try to unit test this instead?)
mapper and linq tests (there are a lot of them)
Example tests that can be improved:
Should_Not_Leak_Connections_Test is taking too long, we can change it to be executing queries at the same time that another thread takes down nodes and then verify at the end the difference in GCMemory